Mark,
My pain doc has told me my battery will do the best if I "top it off every few days" I sometimes forget to do that as I charge mine when I am driving (I am a home health nurse and drive tons of miles daily). My unit is on for the most part unless life is REALLY good then I have been able to have it off for as much as a week. But I am finding the pain seems to take longer to control if I leave it off for more then the time it take to charge.